Several types of flowmeters exist but most of them are expensive, notably if it is desired to measure low flow rates and in a relatively extensive range of flow rates. For example, flowmeters with calibrated ports may be mentioned, the fluid flowing through a port and the pressure difference between the upstream and downstream flow from the port is measured. However, when the fluid is loaded with solid particles, the port may get clogged up or be damaged thereby changing its dimension which distorts the measurement. Flowmeters operating according to Coriolis forces are also known: the fluid flows in a U-shaped tube which is set into oscillation. The change in the oscillation frequency characterizes the mass of the fluid which passes through the tube and therefore the flow rate of the fluid. Ultrasonic flowmeters also exist which operate with the Doppler effect. However, these types of apparatuses are poorly adapted to the measurement of low flow rates.
All these flowmeters have the drawback of being expensive, each costing between 15,000 and 20,000 ε.
Another technology for measuring the flow rate of a fluid consists of locally “marking” the fluid with a sudden change in temperature, by means of a heat source such as for example a heating resistor, and measuring the elapsed time for the thermally marked fluid to travel a certain distance. This distance may either be the distance separating the heat source from a detector placed downstream from this heat source, or the distance separating two detectors located downstream from the heat source. This technique is described in European Patent Application No. 0,289,361, for example. A heating unit encircles a tube in which a liquid flows, the flow rate of which is intended to be measured. The heating unit is periodically powered for brief periods so as to only heat a small amount of liquid, whenever it is powered.
The temperature of the fluid is raised, thereby thermally marking the small amount of liquid. Two infrared radiation detectors, located downstream from the heating unit and spaced apart by a known distance, detect the passage of the heated liquid. A clock connected to these detectors measures the time taken by the heated liquid to travel the distance separating both detectors. The flow velocity of the liquid may therefore be calculated and then the flow rate may be determined. It is noted that the method described in this patent application requires two detectors.
Concerning the thermal marking as described in the prior art, a heat source should, first of all, be available, capable of very rapidly and locally raising the temperature of a small portion of fluid so that the temperature gradient may easily be detected. As an example, raising the temperature of a cubic centimeter of water by one Celsius degree requires 4.2 Watt.s. If is desired to raise the temperature of 50 to 500 cc of water by one to three degrees C., a pulsed mode power of 3.5 to 105 Watt/s needs to be available. This may be achieved by an infrared laser or by microwaves, but with a high cost. Moreover, it may be difficult to control the power of a microwave generator.
Good stability of the boundary separating the areas with different temperatures is also needed for effective thermal marking. Turbulences of the fluid should therefore be avoided and for this purpose, the distance separating both detectors and the distance separating the heat source from the detector should be short.